Handle conflicts with system-installed cargo

[msprotz]

After doing git clean -fdx and rustup update on my Intel OSX machine, build fails for charon:

make
...
error[E0554]: `#![feature]` may not be used on the stable release channel
--> macros/src/lib.rs:4:1
  |
4 | #![feature(non_exhaustive_omitted_patterns_lint)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^



For more information about this error, try `rustc --explain E0554`.
error: could not compile `macros` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
make[1]: *** [Makefile:6: build] Error 101
make[1]: Leaving directory '/Users/jonathan/Code/charon/charon'
make: *** [Makefile:36: build-charon-rust] Error 2
jonathan@absinthe:~/Code/charon (main) $ git rev-parse head
45b95e0f63cb830202c0b3ca00a341a3451a02ba

Let me know if I'm doing anything wrong! Thank you.

[Nadrieril]

We discussed this on zulip, I believe the issue was that cargo was installed with homebrew and shadowed rustup's cargo. Since charon requires a precise version of rust, it can't build with a system-wide installed rustc toolchain. The solution is to uninstall the non-rustup toolchain, or to make sure rustup takes precedence (e.g. in the PATH variable).

[sonmarcho]

Should we close this issue?

[Nadrieril]

I was waiting for @msprotz to confirm that he can build charon now

[msprotz]

Yes it works now that I uninstalled the brew-provided cargo. Feel free to turn this into "warn against using system cargo", or "make sure cargo comes from rustup", or just close it. Thanks!

[Nadrieril]

I opened rust-lang/rustup#3990, we should at least find a way to warn the user.